Method and device for hardening a metal component by plasma pulse technology

ABSTRACT

A process and a device for the pulsed plasma strengthening of a metallic component, in which the surface to be strengthened is not acted on in a merely punctiform manner and the roughness of the surfaces is not adversely effected, comprising the steps of: providing a metallic component ( 1, 8, 18 ), coating a surface section ( 7, 7 ′) of the metallic component ( 1, 8, 18 ) with a sublimable material ( 22 ), applying a pulsed plasma jet ( 20 ) to the surface section ( 7, 7 ′) in such a way that the material is sublimed and as a result a shock wave is introduced into the component ( 1, 8, 18 ) in order to form residual compressive stresses in a region which extends from the surface section ( 7, 7 ′) into the component ( 1, 8, 18 ).

[0001] The invention relates to a process and a device for the pulsedplasma strengthening of a metallic component.

[0002] Components of gas turbines are subject to vibrational loads andare also exposed to mechanical or fluid-dynamic or erosive wear.Internal compressive stresses are introduced into the components by shotpeening or pulsed laser strengthening in order to extend the servicelife. Shot peening has the drawback that the surface of the component isadversely affected in terms of the roughness on account of the impactsof the shot. A drawback of the pulsed laser strengthening process, inaddition to the very poor efficiency of the laser, is the punctiformarea of action of the laser pulse, which is locally restricted to thefocal region of the laser, on the component surface which is to bestrengthened.

[0003] The object of the invention is to provide a process forstrengthening a metallic component which does not merely act inpunctiform manner on the surface to be strengthened and also does notadversely affect the roughness of the surface. Moreover, it is intendedto provide a corresponding device for strengthening a metalliccomponent.

[0004] In terms of the process, according to the invention, the solutionis characterized by the following steps: providing a metallic component,coating a surface section of the metallic component with a sublimablematerial, applying a pulsed plasma jet to the surface section in such away that the material is sublimed and as a result a shock wave isintroduced into the component in order to form residual compressivestresses in a region which extends from the surface section into thecomponent.

[0005] In the process, the component is strengthened at the surface andthe regions close to the surface without the component roughness beingincreased, this being undesirable in particular for components of gasturbines, on account of aerodynamic and strength aspects. The componentsurface is not damaged by the pulsed plasma strengthening. Moreover, theplasma jet, which consists of a mixture of electrons and positivelycharged atomic nuclei (ions) and therefore includes matter, at thesurface of the component acts on a significantly larger area than thefocal region of a laser beam, with the result that the process can becarried out more quickly and more economically. The material which isused to coat the surface of the component which is to be strengthenedsublimes in a short high-energy pulse on account of the conversion ofthe thermal and kinetic energy of the plasma pulse. The introduction ofthis sublimation detonation shock in the form of a shock wave into thecomponent causes the latter to be strengthened or hardened at thesurface and in regions close to the surface down to penetration depthsof approximately 2 mm, on account of microstructural changes.

[0006] Unlike the pure light beam of the laser pulse, the plasma jet isa targeted jet of matter, the energy of which is composed of a kineticcomponent and a thermal component. These components usually each amountto approximately 50%. The kinetic energy component causes the plasma jetto penetrate into the layer of sublimable material and ensures a highlyefficient introduction of energy or power. The sublimation of this layerand the resulting shock wave are then generated through conversion ofthe overall energy of the plasma jet. The kinetic effect, i.e. thedeceleration of the extremely highly accelerated particles of the plasmajet, results in a high pressure being exerted on the layer of sublimablematerial and the component itself, preventing the sublimation gases fromescaping prematurely and making it possible to improve the introductionof the shock wave into the component. The use of what are known ascovering layers or mist, which is required with laser pulses, is notnecessary.

[0007] With special plasma jets, it is possible to introduce permanentcompressive stresses into a region extending from the surface sectioninto the component purely by utilizing the kinetic effect.

[0008] On account of the permanent compressive stresses, the wallthicknesses of components which are subject to vibrational loads andtherefore the weight of these components can be reduced. The shock wavewhich enters the body is generated at the surface of the component as aresult of the action of the pulsed plasma jet on the sublimable materialand the associated detonation-like sublimation.

[0009] The process can be used to strengthen and harden compressorcomponents, such as blades, vanes or discs, made from Ti-based alloysand also components made from Fe- or Ni- or Co-based alloys in regionsclose to the surface.

[0010] The region which has been pulsed plasma strengthened makes itpossible to significantly increase the fatigue strength of componentswhich are subject to vibrational loads, on account of the formation ofpermanent compressive stresses in the regions close to the surface. Theintroduction of compressive stresses into the component by means ofpulsed plasma strengthening also affects the service life of blades,vanes and discs of compressors or turbines of gas turbines which aresubject to vibrational loads. Moreover, the pulsed plasma strengtheningof the regions close to the surface has positive, life-extending effectswith regard to frictional corrosion, known as fretting, which occurs atthe contact points between blade roots and receiving grooves in discs.

[0011] The sublimable material is preferably organic, e.g. athermoplastic, such as PVC, PE, PP, and acts as a energy-transmittingmedium, via which the sublimation detonation shock which is formed whenthe pulsed plasma jet impinges is introduced into the component as ashock wave.

[0012] The plasma jet is applied areally to the surface section of thecomponent, it being possible for the plasma jet to act on an area ofapproximately 10 cm² and larger per plasma pulse. This value can bevaried by means of the power data and the geometry of the pulsed plasmasource used and is matched to the geometry of the surface section whichis to be strengthened.

[0013] On account of the highly efficient introduction of energy andpulses into the layer of sublimable material, the component can bestrengthened or hardened down to penetration depths of approximately 2mm as a result of microstructural changes brought about by the shockwaves.

[0014] The surface section can be exposed to a plasma jet with avelocity of at least approximately 150 km/s, so that the particles ofthe plasma jet penetrate into the layer of sublimable material and donot rebound.

[0015] The pulsed plasma jet can be generated on the basis of a pulsedhigh-current arc discharge, in which case the working gas which isionized by the arc, such as He or Ar, emerges from a pulsed plasmasource in the form of a detonation-like plasma jet.

[0016] The process is preferably carried out under a vacuum in theregion of 10⁻⁷ bar, in order to avoid energy losses from the plasma jetand therefore a reduction in efficiency.

[0017] The pulsed plasma strengthening process can also be usedeffectively for components of gas turbines or the like which havealready been subject to damage in operation, in the form of cracks ornotches. With components of this type, a surface section which lies inthe region of the damage is exposed to the plasma jet so that permanentcompressive stresses are introduced into the component, with the resultthat further crack propagation is prevented and the remaining servicelife of this component can be increased to the level of new parts.

[0018] With regard to the device, according to the invention thesolution is characterized by a pulsed plasma source having a volume forholding working gas and a nozzle which is directed towards a metallicsubstrate, holding and positioning means for the pulsed plasma sourceand the substrate, a means for coating the substrate with a sublimablematerial, a means for discontinuously feeding working gas into thevolume and a means for introducing extremely high power into the plasmagas in order to generate a pulsed plasma jet which emerges in the mannerof a detonation. In this case, the particles of the plasma jet areaccelerated to ultrasound velocity.

[0019] In order to bring about the extremely high introduction of powerof an order of magnitude of 10 GW for the detonation-like emergence ofthe plasma from the pulsed plasma source, the energy is preferablyintroduced by means of high-current arc discharge.

[0020] The means for discontinuously feeding working gas into the volumepreferably comprises at least one valve, which is controlled in such away that new working gas is supplied immediately after thedetonation-like emergence of plasma gas.

[0021] The pulsed plasma jet may emerge at frequencies in the range from1-10 Hz, depending on the capacitor charging unit and the valve.

[0022] Further configurations of the invention are described in thesubclaims.

[0023] In the text which follows, the invention is explained in moredetail on the basis of exemplary embodiments and with reference to adrawing, in which:

[0024]FIG. 1 shows a perspective illustration of a blade or vane of agas turbine, the surface of which, at least in sections, can be pulsedplasma strengthened using the process according to the invention,

[0025]FIG. 2 shows a perspective illustration of a rotor of a gasturbine, which substantially comprises a disc with a hub and blades, and

[0026]FIG. 3 diagrammatically depicts a device with a pulsed plasmasource.

[0027]FIG. 1 shows a blade or vane of a gas turbine which is denotedoverall by 1 and the surfaces or regions close to the surface of whichare strengthened and hardened at least in sections by plasma pulses, soas to introduce permanent compressive stresses. The pulsed plasmastrengthening process can be used both for blades or vanes 1 ofcompressors and of turbines. When a gas turbine is operating, the bladeor vane 1 is subject to particularly high vibrational loads at thetransition from an inner platform 3 to the blade leaf 2 or to the bladeroot 4. In addition, the blade root 4 is also subject to frictionalcorrosion or fretting.

[0028] Furthermore, the surfaces of the gas turbine component in theflow duct are exposed to fluid-dynamic wear or erosion from particles,contaminants or the like, the greatest potential for damage lying in theregion of a leading edge 5 of the blade or vane 1. The regions of theblades or vanes 1 which sweep past the housing of the gas turbine, suchas for example blade or vane tips 6 or sealing tips or sealing fins (notshown) on blades or vanes with a cover strip, are exposed to mechanicalwear and can be treated in the same way by pulsed plasma strengtheningwith a view to being reinforced.

[0029] By way of example, a surface section 7 is indicated by dashedlines in the region of the leading edge 5 of the blade or vane 1 and canbe pulsed-plasma strengthened in accordance with an exemplary embodimentof the process according to the invention, in order to introducepermanent compressive stresses in a region which extends from thesurface section 7 into the component 1 and in this way to strengthen thecomponent 1. A further surface section 7′, which lies in the region ofthe blade or vane tip 6 and is likewise indicated by dashed lines, canbe pulsed plasma strengthened in accordance with an exemplary embodimentof the process according to the invention, in order to mechanicallyreinforce the blade or vane at that location and to protect it againstwear during stripping.

[0030] Detachable blades or vanes 1 of this type are secured in apositively locking manner to discs with numerous longitudinal grooves orone encircling circumferential groove. Like the root 4 of the blade orvane 1, grooves of this type (not shown) in discs are regions which arecritical in terms of stresses and are at risk of fracture and which canbe pulsed plasma strengthened in order to increase the service life.

[0031] In the process for pulsed plasma strengthening, the blade or vane1, as diagrammatically depicted in FIG. 3 for a component 18, is fixedin a holding and positioning means in such a way that a pulsed plasmasource 14 can apply a pulsed plasma jet 20 to the surface section 7, 7′which is to be strengthened. Before this, the surface section 7, 7′ iscoated with a layer 22 of sublimable material, such as for example athermoplastic, such as PVC, of a suitable thickness. The thickness ofthe layer 22 is dependent, inter alia, on the material of the component1, 8, 18.

[0032] Then, a working gas, e.g. He, H₂ or Ar, is fed to a volume 17 ofthe pulsed plasma source 14, and power is fed to this working gas bymeans of a means for introducing extremely high power in the region of10 GW, such as a high-current arc discharge 15, 16, within an extremelyshort time. The result is the formation of a pulsed plasma beam 20 whichemerges in the manner of a detonation from a nozzle 21 of the pulsedplasma source 14 and penetrates into the sublimable material 22 withwhich the surface section 7, 7′ has been coated, and the total energy ofwhich plasma beam, which includes thermal and kinematic components,causes the material 22 to sublime in a short high-energy pulse. Onaccount of the high pressure on the layer resulting from thedeceleration of the extremely highly accelerated particles of the plasmajet 20, this sublimation detonation shock is introduced directly intothe component 1 and leads to a shock wave, as a result of which thecomponent is strengthened and hardened down to penetration depths ofapproximately 2 mm on account of microstructural changes. In addition toan initial shock wave, reflected shock waves also occur.

[0033] On account of the large area of action of the plasma jet 20 onthe surface section 7, 7′ coated with sublimable material 22, this areaamounting to approximately 10 cm² or larger, the pulsed plasmastrengthening proves to be an extremely economical process. Even surfacesections 7, 7′ which are narrow or difficult to gain access to can bepulsed plasma strengthened by suitably selecting the holding andpositioning means for the pulsed plasma source 14 and the substrate 1,18.

[0034]FIG. 2 shows an integrally bladed rotor, which is denoted overallby 8 and substantially comprises a disc 9 with a hub 10 and acircumferential surface 11 as well as substantially radially extendingblades 12. The blades 12 of an integrally bladed rotor 8 are subject todynamic loads not only in the critical regions which have already beendescribed in connection with the blade or vane 1 illustrated in FIG. 1but also in particular in the transition region from the blade leaf tothe circumferential surface 11 of the disc 9. In the disc 9, the holeregion of the hub 10 is subject to particularly high loads, on accountof the high tensile stresses caused by the centrifugal force.

[0035] The critical surface sections of the rotor 8 in the transitionregion between a blade 12 and the circumferential surface 11 of the disc9 and the hole region of the hub 10 are pulsed plasma strengthenedsubstantially in accordance with the process steps which have beendescribed in connection with the blade or vane 1 shown in FIG. 1. Firstof all, a layer 22 of sublimable, generally organic material, such asfor example a thermoplastic, is applied to the surface section which isto be strengthened, corresponding to 7, 7′ in FIG. 1. For each plasmajet 20, a working gas, such as Ar, is fed to a pulsed plasma source 14,the nozzle 21 of which is directed onto the surface section which is tobe strengthened, and a means 15, 16, such as a high-current arcdischarge, is used to supply an extremely high power to the working gasin order to generate the plasma jet 20, which comprises electrons andpositively charged atomic nuclei (ions) and therefore includes matter.The pulsed plasma source 14 operates at a frequency in the range from1-10 Hz, so that the plasma jet 20 emerges from the pulsed plasma source14 in the manner of a detonation and in this way impinges on andpenetrates into the layer of sublimable material 22.

[0036] The layer of sublimable material 22 serves as anenergy-transmitting medium in such a manner that the sublimationdetonation shock which results from the sublimation in a shorthigh-energy pulse is introduced directly into the region of the rotor 8which is to be strengthened, where a shock wave is formed, causing theregion in the rotor 8 to be strengthened and hardened on account ofmicrostructural changes.

[0037] By suitably selecting the holding and positioning means for thepulsed plasma source 14 and the rotor 8, it is also possible forinterior cavities, such as the hole region of the hub 10 of the rotor 8,which is subject to high loads, to be pulsed plasma strengthened usingthe processes described without any difficulty.

[0038] Components which have already been damaged in operation, such asfor example a blade or vane 1 shown in FIG. 1 or an integrally bladedrotor 8 shown in FIG. 2 or torque-transmitting shafts, in which wearoccurs in the form of notches, cracks or fretting, can have theirremaining service life increased to the level of undamaged new parts bymeans of pulsed plasma strengthening of surface sections which lie inthe region of the damage. Particularly in the case of cracks or notches,propagation of the damage is prevented by the permanent compressivestresses which are introduced into the surface section by the pulsedplasma strengthening and extend into the component down to a depth ofapproximately 2 mm.

[0039] In the case of components which are damaged by cracks or thelike, the surface section 7, 7′ which is to be strengthened and istherefore to be provided with a layer of sublimable material 22 liesaround this damage. The subsequent action of a plasma jet 20 causespermanent compressive stresses, which prevent further crack propagation,to be introduced by the shock wave which has been introduced into thecomponent, e.g. the blade or vane 1.

[0040]FIG. 3 shows a simplified illustration of a device for carryingout the process for the pulsed plasma strengthening of a metalliccomponent 18, which substantially comprises a pulsed plasma source,which is denoted overall by 14 and is based on a pulsed high-current arcdischarge with a variable pulse energy, pulse width and pulse sequencefrequency which can be adapted to the particular application. Theprocess is carried out discontinuously using a plurality of pulsedplasma jets 20. Should the area of the region of the component 18 whichis to be strengthened be larger than the area of action of the plasmajet 20 on the component surface, the pulsed plasma source 14 and thecomponent 18 are moved with respect to one another during the process,in such a way that the whole of the desired surface section can beexposed to the pulsed plasma jets 20.

[0041] Particularly in the case of relatively large surface sections 7,7′ which are to be strengthened, it is additionally possible to use whatis known as a magnetic nozzle, in which the plasma jet 20 is controlled,diverted or shaped using a magnetic field in order to achieve a uniformenergy density. In this way it is also possible, for example, togenerate annular plasma jets 20 for example for the pulsed plasmastrengthening of discs.

[0042] The pulsed plasma source 14 comprises two cooled electrodes 15,16, between which an arc discharge with an extremely high energy densityis generated. The working gas used is a noble gas or a chemicallyreactive gas, such as for example He, H₂ or Ar. The working gas is fedvia valves 19 to a volume 17 of the pulsed plasma source 14 which isopen towards the metallic component 18 which is to be pulsed plasmastrengthened at least in sections. Depending on the particularapplication, the component 18 is positioned at a distance in the rangefrom 20 cm to 1 m from the pulsed plasma source 14.

[0043] The arc which is generated emits heat to the working gas, whichis ionized so as to form a plasma and leaves the pulsed plasma source 14at the nozzle 29 in the manner of a detonation in the form of a pulsedplasma jet 20 which is indicated by arrows and is at an extremely hightemperature and velocity. The plasma jet 20 is a mixture of electronsand positively charged atomic nuclei (ions) and includes approximately50% kinetic energy and 50% thermal energy. When the plasma jet 20impinges on and penetrates into the layer 22 of thermoplastic which hasbeen applied to the surface section, e.g. 7, 7′, of the component 18which is to be strengthened by means of plasma pulses, the plasticsublimes in a short high-energy pulse and transmits the sublimationdetonation shock in the form of a shock wave into the component 18. Thelayer 22 therefore serves as an energy-transmitting medium forintroducing the sublimation detonation shock into the component 18.

[0044] Immediately after the detonation-like emergence of the plasma gas20 from the pulsed plasma source 14, further working gas is fed to thepulsed plasma source 14 via the valves 19, and this gas is in turnheated by the arc between anode and cathode 14 and 15, respectively, andemerges in the manner of a detonation through the nozzle 21 towards thecomponent 18. The frequency is in the range from 1-10 Hz.

[0045] The arc of the pulsed plasma source 14 is not transmitted ontothe component 18, so that the component remains relatively cool duringthe process even though, depending on the particular application, it maybe arranged at only a short distance from the nozzle 21 of the pulsedplasma source 14.

[0046] The process and device for the pulsed plasma strengthening canalso be applied to other metallic components of thermal, power orworking machines in which regions which extend from the surface into thecomponent are to be strengthened or hardened by the introduction ofpermanent compressive stresses.

1. Process for the pulsed plasma strengthening of a metallic component,comprising the steps of: providing a metallic component (1, 8, 18),coating a surface section (7, 7′) of the metallic component (1, 8, 18)with a sublimable material (22), applying a pulsed plasma jet (20) tothe surface section (7, 7′) in such a way that the material is sublimedand as a result a shock wave is introduced into the components (1, 8,18) in order to form residual compressive stresses in a region whichextends from the surface section (7, 7′) into the component (1, 8, 18).2. Process according to claim 1, characterized in that a component (1,8, 18) made from a Ti- or Fe- or Ni- or Co-based alloy.
 3. Processaccording to claim 1 or 2, characterized in that a component (1, 8, 18)of a gas turbine which is subject to vibrational loads is provided. 4.Process according to claim 3, characterized in that a blade or vane (1)or a disc (9) of a compressor or a turbine is provided.
 5. Processaccording to one or more of the preceding claims, characterized in thatthe sublimable material is organic.
 6. Process according to one or moreof the preceding claims, characterized in that the sublimable materialis a plastic, preferably a thermoplastic.
 7. Process according to one ormore of the preceding claims, characterized in that the plasma jet (20)is applied areally to the surface section (7, 7′).
 8. Process accordingto claim 7, characterized in that the plasma jet (20) in the surfacesection (7, 7′) acts on an area of approximately 10 cm² per plasmapulse.
 9. Process according to one or more of the preceding claims,characterized in that the residual compressive stresses are introducedin a region which extends from the surface section (7, 7′) as far as 2mm into the component (1, 8, 18).
 10. Process according to one or moreof the preceding claims, characterized in that the pulsed plasma jet(20) is generated capacitively or inductively using high-frequency fieldor on the basis of a pulsed high-current arc discharge.
 11. Processaccording to one or more of the preceding claims, characterized in thatthe surface section (7, 7′) is exposed to a pulsed plasma jet (20) whichincludes matter.
 12. Process according to claim 11, characterized inthat the surface section (7, 7′) is exposed to a pulsed plasma jet (20)which has thermal and kinetic energy.
 13. Process according to one ormore of the preceding claims, characterized in that the surface section(7, 7′) is exposed to a plasma jet (20) which impinges on it in themanner of a detonation.
 14. Process according to one or more of thepreceding claims, characterized in that the surface section (7, 7′) isexposed to a plasma jet (20) which is pulsed in a frequency in the rangefrom 1-10 Hz.
 15. Process according to one or more of the precedingclaims, characterized in that the surface section (7, 7′) is exposed toa plasma jet (20) with a velocity of at least approximately 150 km/s.16. Process according to one or more of the preceding claims,characterized in that the surface section (7, 7′) is exposed to theplasma jet (20) under a vacuum.
 17. Process according to one or more ofthe preceding claims, characterized by: providing a damaged metalliccomponent (1, 8, 18) and exposing the surface section (7, 7′) which liesin the region of the damage to the plasma jet (20).
 18. Device for thepulsed plasma strengthening of a metallic component, comprising a pulsedplasma source (14) having a volume (17) for holding working gas and anozzle (21) which is directed towards a metallic substrate (18), holdingand positioning means for the pulsed plasma source (14) and thesubstrate (18), a means for coating the substrate (18) with a sublimablematerial, a means (19) for discontinuously feeding working gas into thevolume (17) and a means (15, 16) for introducing extremely high powerinto the working gas in order to generate a pulsed plasma jet (20) whichemerges in the manner of a detonation.
 19. Device according to claim 18,characterized in that the introduction of power into the working gas iseffected capacitively or inductively by means of a high-frequency fieldor by means of high-current arc discharge.
 20. Device according to claim18 or 19, characterized in that the feed means for the working gascomprises at least one valve (19).
 21. Device according to one or moreof the preceding claims 18 to 20, characterized in that the pulsedplasma jet (20) emerges at a frequency of 1-10 Hz.
 22. Device accordingto one or more of the preceding claims 18 to 21, characterized in thatthe nozzle (21) has a diameter in the range from 50-500 mm.
 23. Deviceaccording to one or more of the preceding claims 18 to 22, characterizedby an efficiency in the range from 40% to 50%.